1. Field of the Invention
The present invention relates to an FM radar system for use as a distance/speed detector in a collision alarm system on motor vehicles, and more particularly to a multibeam FM radar system for radiating a plurality of FM radar beams in respective different directions with adjacent ones of the FM radar beams overlapping each other.
2. Description of the Prior Art
FM millimeter-wave radar devices for use as distance/speed detectors in collision alarm systems on motor vehicles are known from "Radar technology" published by the Society of Electronic Information Communications. The known FM millimeter-wave radar devices radiate a signal whose frequency increases or decreases in a triangular wave pattern with time and receive an echo signal reflected by another motor vehicle. The received echo signal is mixed with the transmitted signal, generating a beat signal. The distance up to the motor vehicle and the speed thereof are detected from the frequency f of the beat signal (beat frequency f).
FIG. 1A of the accompanying drawings shows the frequency of an FM signal radiated from an FM radar system on a moor vehicle and the frequency of an echo signal reflected by another motor vehicle and received by the FM radar system. The radiated and received signals have a frequency which varies in a triangular wave pattern with time. In FIG. 1A, it is assumed that the motor vehicles are running at the same speed, i.e., the relative speed between the motor vehicles is nil. As shown in FIG. 1A, while the frequency of the radiated FM signal is linearly increasing in a rising period, the frequency of the received echo signal which appears with a time lag is lower than the frequency of the radiated FM signal. On the other hand, while the frequency of the radiated FM signal is linearly decreasing in a falling period, the frequency of the received echo signal which appears with a time lag is higher than the frequency of the radiated FM signal. If the motor vehicles are running at respective different speeds, i.e., the relative speed between the motor vehicles is not nil, then as shown in FIG. 1B of the accompanying drawings, a Doppler shift fp depending on the relative speed between the motor vehicles is introduced into the beat frequency f that is generated if the relative speed between the motor vehicles is nil.
The Doppler shift fp affects in opposite directions the beat frequency fu detected during the rising period of the frequency of the radiated FM signal and the beat frequency fd detected during the falling period of the frequency of the radiated FM signal. Specifically, these beat frequencies fu, fd are given as follows: EQU fu=f-fp (1) EQU fd=f+fp (2)
From the above equations (1), (2), the following equations are obtained: EQU f=(fu+fd)/2 (3) EQU fp=(fu-fd)/2 (4)
The distance R between the motor vehicles and the relative speed u between the motor vehicles are expressed as follows: EQU R=cf/(4fm.multidot..DELTA.f) (5) EQU u=cfp/2fo (6)
where c is the speed of light, .DELTA.f the range across which the frequency of the transmitted FM signal is variable, fm the period at which the frequency of the transmitted FM signal is variable, and fo is the central frequency of the transmitted FM signal.
The beat frequency is usually detected when the beat frequency is subjected to a fast-Fourier transform (FFT). FIG. 1C of the accompanying drawings shows a frequency spectrum of the beat signal produced by the fast-Fourier transform. As shown in FIG. 1C, a pair of beat frequencies (fu, fp) in the rising and falling periods, which are shifted by the Doppler shift fp from the beat frequency f that is generated if the relative speed between the motor vehicles is nil, appears on opposite sides of the beat frequency f.
If there is only one motor vehicle which reflects an echo signal to the FM radar system, then the distance up to the motor vehicle and the relative speed with respect thereto can easily be detected from the above pair of beat frequencies and the equations (3).about.(6). However, if there are plural motor vehicles which reflect respective echo signals to the FM radar system, then the FM radar system detects a plurality of beat frequencies, and needs to carry out a complex process for making proper pairs of these beat frequencies, i.e., pairing the beat frequencies.
According to one conventional pairing process proposed in Japanese laid-open patent publication No. 5-142337, beat frequencies in the rising and falling periods are sequenced in the order of frequencies, and paired according to the sequence to recognize motor vehicles that have reflected echo signals for thereby detecting the distances up to the motor vehicles and the relative speeds with respect thereto. When a plurality of motor vehicles run closely to each other, however, beat frequencies produced with respect to those motor vehicles tend to switch around due to Doppler shifts and spectral fluctuations. In such a situation, the motor vehicles cannot easily be distinguished only on the basis of the sequence of the beat frequencies.
It often happens in reality that two beat frequencies that have been generated in relation to different motor vehicles are superposed to the extent that they appear as a single beat frequency on the frequency spectrum. Since it is difficult to distinguish those overlapping beat frequencies from each other, beat frequencies that are involved cannot easily be paired. One proposal revealed in Japanese laid-open patent publication No. 5-150035 is that missing data is compensated for by past data based on the linear prediction that the speeds of motor vehicles are constant. However, when the speed of a motor vehicle changes abruptly or one of paired beat frequencies is not detected for a long period of time, a distance that has been made up for by past data is apt to be widely different from the actual distance. Another problem is that a motor vehicle which suddenly enters the range of the radar system upon a lane change cannot be compensated for because no past data is available for that motor vehicle.